The present invention relates to half-duplex data transmission.
Conventional half-duplex data sets operate in a switched carrier mode. In accordance with this mode of operation, when a "near-end" data set detects carrier energy from the far-end data set, it maintains its own carrier off and, at the same time, raises a Received Line Signal (RSL) lead extending to the associated near-end terminal equipment. The raised RSL lead serves as an indication to the near-end terminal equipment that data originating from the far end is being received and that the near-end equipment should inhibit itself from attempting to transmit data to the far end.
Once the far-end terminal has completed its transmission session, the far end data set turns off its carrier. The near-end data set responds to the disappearance of carrier by dropping its RLS lead. The near-end terminal equipment, if it has data to send, can now request the opportunity to do so by raising its Request-to-Send (RTS) lead extending to the near-end data set. The latter, in response, turns on its carrier and, after a predetermined delay which is discussed below, raises its Clear-to-Send lead extending back to the near-end terminal equipment. The near-end equipment now begins passing its data to the near-end data set, which modulates it onto the now-on, near-end carrier. Upon detecting this carrier energy, the far-end data set raises its RSL lead, thereby inhibiting the far-end terminal equipment from raising its RTS lead, and so forth.
An important parameter in the operation of half-duplex data sets is the so-called turnaround time, or delay, this being the minimum time in which one data set can raise its CTS lead after the other has dropped its RTS lead. The principal components of the turnaround time are (a) the time required for a data set to detect the disappearance of carrier energy from the channel (and thereupon drop its RLS lead), and (b) the above-mentioned predetermined delay, whose duration is set to be equal to the time that will be required after that data set has turned on its own carrier for the other data set to detect the presence of carrier and to acquire carrier phase and sample timing. The turnaround time is typically not insubstantial--a duration of 155 ms being typical for two-wire data sets such as the AT&T 201C data set--and may have a substantial negative impact on data transmission throughput--particularly in applications in which the direction of data transmission changes frequently.
In contrast to half-duplex data sets, full duplex data sets typically do not have an associated turnaround time. This is because full-duplex data sets transmit and receive over separate transmission channels (which are either physically distinct or, for example, frequency-divided portions of a single physical channel). Each data set maintains its carrier on continuously, thereby obviating the need to wait for the other data set to detect carrier energy and acquire carrier phase and sample timing.
The use of full-duplex data sets is not always a practical solution to the avoidance of long turnaround delays, however, because much existing data processing/communications equipment is designed to work using a half-duplex protocol. That is, the equipment relies on receiving an RLS signal to tell it whether, on the one hand, it is free to raise its RTS lead to initiate a transmission, in which case it will ignore any incoming data, or whether, on the other hand, incoming data is being received, in which case the terminal will inhibit itself from raising its RTS lead.